Object-oriented Software in Ada 95

Polymorphism 243
The method Add_Observer adds the access value of an observer to the array Viewers. The exception
Constraint_Error is raised if this operation cannot be accomplished due to lack of space.
procedure Add_Observer( The:in out Observable;
O:in P_Observer ) is
begin
for I in 1 .. The.Last loop --Check for empty slot
if The.Viewers( I ) = null then
The.Viewers( I ) := O;
--Populate
return;
end if;
end loop;
if The.Last >= Viewers_Index'Last then --Extend
raise Constraint_Error; -- Not enough room
else
The.Last := The.Last + 1; -- Populate
The.Viewers( The.Last ) := O;
end if;
end Add_Observer;
The inverse method Delete_Observer removes an observer for the observed object.
procedure Delete_Observer( The:in out Observable;
O:in P_Observer ) is
begin
for I in 1 .. The.Last loop --For each observer
if The.Viewers( I ) = O then --Check if to be removed
The.Viewers( I ) := null;
end if;
end loop;
end Delete_Observer;
When the model of the object (the observed) has changed and it is required to redisplay a representation of it
the method Notify_Observers is called. This calls the Update method in each observer.
procedure Notify_Observers( The:in out Observable'Class ) is
begin
for I in 1 .. The.Last loop --For each observer
if The.Viewers( I ) /= null then -- call it's
Update( The.Viewers( I ).all, The );-- update method
end if;
end loop;
The.State_Changed := True; --
end Notify_Observers;
Note:
The second parameter is the observed object that is to be displayed by the Update method.
The method Set_Changed simply records that the state of the observed object has changed.
procedure Set_Changed( The:in out Observable ) is
begin
The.State_Changed := True;
end Set_Changed;
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